3406pk
Well-Known Member
I have had a stumble or hesitation that comes on at 3500-4000 rpm on my 408 sb duster.
First the specs: 5.9L Dodge engine with stroker, 408 ci, 10.5 cr, Hughes hyd. roller cam 240I/242E, Eddy RPM airgap intake, Holley 870 street avenger (vacuum secondary). Accel distributor, Rev-n-nator ign control box, timing 18 initial, 34 total, Champion plugs 3344. Trans 727, 3.55 rear gear.
I have taken the car to 1/8 mile drags for about 5 years. Best 7.95 at 91 mph. Was pretty consistent until end of last season. Then it developed hestitation as noted above, in 1st gear would go thru it pretty fast, but 2nd gear was noticeable. Did not do it all time and thought at first it was heat related like vapor lock. Added 1" plastic carb spacer, but didn't make much change. So at Devils Run car show this May there was a shop doing chassis dyno short HP pulls for bragging purposes. Anyway I put my car on with the Air/Fuel meter hooked up. Sure enough they could load it so the stumble was very prominent. The engine went very rich down to 9 to 10 on A/F ratio. We didn't have much time or parts to chase it and mechanic thought maybe Power Valve was bad or needle seats leaking. I got carb kit and tore it apart. The carb was untouched from when built and dyno'ed in 2007. It has 78 primary jets, 8.5 power valve, 82 secondary jets, 4.5 secondary PV. Yellow spring in vacuum secondary. I found the power valves work ok, but decided to replace primary PV with 6.5 since engine vacuum runs 13 and drops to 11 in gear. I thought about going to 80 secondary jets and 77 on primary. Some plugs are tan and most are sooty. I was also going to try Autolite 65 plugs since some members have good luck with them. I am checking plug wires, and ignition for problems too.
Attached is dyno sheet and the dip in A/F and HP are really noticeable. Original engine dyno tests was little rich but not that bad. Is there some tuning tips that would help that dip. Thanks for your expertise.
First the specs: 5.9L Dodge engine with stroker, 408 ci, 10.5 cr, Hughes hyd. roller cam 240I/242E, Eddy RPM airgap intake, Holley 870 street avenger (vacuum secondary). Accel distributor, Rev-n-nator ign control box, timing 18 initial, 34 total, Champion plugs 3344. Trans 727, 3.55 rear gear.
I have taken the car to 1/8 mile drags for about 5 years. Best 7.95 at 91 mph. Was pretty consistent until end of last season. Then it developed hestitation as noted above, in 1st gear would go thru it pretty fast, but 2nd gear was noticeable. Did not do it all time and thought at first it was heat related like vapor lock. Added 1" plastic carb spacer, but didn't make much change. So at Devils Run car show this May there was a shop doing chassis dyno short HP pulls for bragging purposes. Anyway I put my car on with the Air/Fuel meter hooked up. Sure enough they could load it so the stumble was very prominent. The engine went very rich down to 9 to 10 on A/F ratio. We didn't have much time or parts to chase it and mechanic thought maybe Power Valve was bad or needle seats leaking. I got carb kit and tore it apart. The carb was untouched from when built and dyno'ed in 2007. It has 78 primary jets, 8.5 power valve, 82 secondary jets, 4.5 secondary PV. Yellow spring in vacuum secondary. I found the power valves work ok, but decided to replace primary PV with 6.5 since engine vacuum runs 13 and drops to 11 in gear. I thought about going to 80 secondary jets and 77 on primary. Some plugs are tan and most are sooty. I was also going to try Autolite 65 plugs since some members have good luck with them. I am checking plug wires, and ignition for problems too.
Attached is dyno sheet and the dip in A/F and HP are really noticeable. Original engine dyno tests was little rich but not that bad. Is there some tuning tips that would help that dip. Thanks for your expertise.
